A comparison of three photosensitizers with respect to efficiency of cell inactivation, fluorescence quantum yield and DNA strand breaks.

Intracellular properties of three photosensitizers relevant to photodynamic cancer therapy were compared using cultured human NHIK 3025 cells. When taken up in the cells, the hydrophilic photosensitizer aluminum phthalocyanine tetra sulfonate required about 10 times more quanta of light absorbed per cell to kill 90% of the cells than did the hydrophobic dyes Photofrin II and tetra(3-hydroxyphenyl)porphyrin. In spite of this, the phthalocyanine molecule was the most efficient dye per quantum of excitation light, since the extinction coefficient of the phthalocyanine is more than 10 times higher than that of the two hydrophobic photosensitizers at therapeutic wavelengths. The two hydrophobic dyes had significantly higher fluorescence quantum yields when taken up by cells than when bound to human plasma or human serum albumin, whereas the opposite was true for the hydrophilic phthalocyanine dye--suggesting intracellular aggregation. Finally, the potential genetic toxicities of the drugs in the form of DNA strand breaks were compared. The aluminum phthalocyanine tetra sulfonate photosensitized more DNA strand breaks than did Photofrin II and tetra(3-hydroxyphenyl)porphyrin when compared at the same level of cell survival.